While eye to hand co-ordination and reaction is relatively slow and subject to variability and improvement from practice, and eye to voice reaction time is even slower, the reaction time of the eye itself to stimulus is extremely fast in humans and primates. The eye muscles reflexively react to stimuli without the need for conscious action by the subject. Although this reflex can be consciously overridden, the nature of the stimulus and prior fixation can be engineered by methods disclosed in this invention to ensure that the reliability exceeds 97 percent. Furthermore, because the eye reflex is inherently faster than eye-hand or eye-voice reaction times, any variability in the response has a far lower impact on the accuracy of a reaction dependent measurement. This allows the apparatus to exploit the time information in a variety of ways to increase the data obtainable from each individual test point.
The invention, which is defined in the appended claims, is based on the use of an eye position-measuring device capable of measurement of eye position at intervals of less than 45 ms, of which several types are commercially available, in conjunction with a display unit capable of displaying a multiplicity of visual stimuli and capable of accurate calibration of luminance sufficient to exceed the desired accuracy of the desired test. The device is configured to detect the rapid motion of the eye (known as a saccade) towards a new stimulus and to use this saccade to determine the moment the subject's visual reflex responds to the stimulus. Since the subject need not consciously respond to the stimuli the entire field of view measurement process can be automated. By way of example, a set of stimuli can be presented, each stimulus initially below expected threshold increasing in brightness until the stimulus triggers the reflex saccade of the eye from a fixation stimulus. The time the reflex saccade is detected is used to determine the threshold of the retina for that point. The eye position-measuring device can in a preferred embodiment be used to check that the eye's saccade did in fact occur in the correct direction confirming that the test stimulus and not another distraction caused the saccade. At the moment of the said saccade the stimulus that was the saccade target transforms into the fixation point for the next stimulus. This is an important feature for two reasons.
First, the accuracy of immediate post saccade fixation has been shown to be consistently many times better than long term fixation on a single point, and secondly the visual process of saccading from one stimulus to another in sequence is the normal visual scanning mode of the human and higher primate eye, hence the experience for the patient feels natural and unforced, especially if the frequency of the induced saccade is designed to be equivalent to the normal scanning saccade frequency of the eye. This normal scanning frequency varies from time to time in a given individual and from individual to individual but the invention also discloses a method that allows the practitioner to quickly determine this value accurately. Setting the saccade frequency perfectly is not generally necessary but will help to make the test more accurate particularly with anxious patients.
A major advantage of this method of field of view measurement over the prior art is that it eliminates the need for very large samples to be gathered for each stimulus position and repetitive confirmation of the subject's observation of the stimulus and the reliability of their visual fixation. This vastly reduces the time needed for a diagnostician to establish a subject's field of view.
In preferred forms, the invention exploits a detailed computer model of the human visual system's autonomic reflex timings and uses a response interpolator based on this model to allow more accurate interpretation and extrapolation from data while ensuring that the conditions of the test more closely approximate normal visual tasks. This improves both the comfort of the subject and accuracy of the test results. The invention allows sufficient accuracy to determine progression from one test to another of a fraction of a percent, takes little clinical time to administer and the apparatus itself is economic and easily affordable.
In addition to the above benefits the nature of the disclosed method and apparatus also has utility in diagnosis of other visual disorders not directly related to visual field but still dependent on the exploitation of the computer reflex model. This allows the invention to be applied to the diagnosis of high function visual disorders such as dyslexia and visual “neglect”. Dyslexia is a higher brain function disorder, which can be improved by appropriate training, and “neglect” is a symptom of a particular form of brain damage.